[MLIR][Doc] Update the pass infra doc to advise against let constructor = (NFC)

We should avoid specifying it manually and instead rely on TableGen, see also
cleanups in #127403

Author: joker-eph

mlir/docs/PassManagement.md

@@ -809,11 +809,6 @@ def MyPass : Pass<"my-pass", "ModuleOp"> {
     its various constraints and behavior.
   }];
 
-  // A constructor must be provided to specify how to create a default instance
-  // of MyPass. It can be skipped for this specific example, because both the
-  // constructor and the registration methods live in the same namespace.
-  let constructor = "foo::createMyPass()";
-
   // Specify any options.
   let options = [
     Option<"option", "example-option", "bool", /*default=*/"true",
@@ -883,8 +878,7 @@ struct MyPassOptions {
 #endif // GEN_PASS_DECL_MYPASS
 ```
 
-If the `constructor` field has not been specified in the tablegen declaration,
-then autogenerated file will also contain the declarations of the default
+The utogenerated file will also contain the declarations of the default
 constructors.
 
 ```c++
@@ -927,9 +921,8 @@ struct MyPass : foo::impl::MyPassBase<MyPass> {
 These definitions can be enabled on a per-pass basis by defining the appropriate
 preprocessor `GEN_PASS_DEF_PASSNAME` macro, with `PASSNAME` equal to the
 uppercase version of the name of the pass definition in tablegen.
-If the `constructor` field has not been specified in tablegen, then the default
-constructors are also defined and expect the name of the actual pass class to
-be equal to the name defined in tablegen.
+The default constructors are also defined and expect the name of the actual pass
+class to be equal to the name defined in tablegen.
 
 Using the `gen-pass-doc` generator, markdown documentation for each of the
 passes can be generated. See [Passes.md](Passes.md) for example output of real
@@ -951,12 +944,14 @@ contains the following fields:
 *   `dependentDialects`
     -   A list of strings representing the `Dialect` classes this pass may
         introduce entities, Attributes/Operations/Types/etc., of.
-*   `constructor`
-    -   A code block used to create a default instance of the pass.
 *   `options`
     -   A list of pass options used by the pass.
 *   `statistics`
     -   A list of pass statistics used by the pass.
+*   `constructor`
+    -   A code block used to create a default instance of the pass.
+        Specifying it will disable the constructors auto-generation for the
+        pass. This is a legacy option, it is not advised to use it.
 
 #### Options